Our capacity for complex social interactions is a defining feature of humanity, but how did it evolve? It seems like it would have been a slow, gradual process, but a new statistical model suggests something very unusual happened 52 million years ago.

The last few decades have seen remarkable strides in our understanding of social evolution in species like bees and birds. In these species, it appears that complex social structures develop gradually in many steps. Once solitary individuals will first pair off with each other, or live with a small group of their own offspring. It's from these initial, family-like units that eventually larger and much more complex societies emerge, and the key point is that you can't go straight from one individual doing its own thing to an entire complex, interconnected society.

And yet, according to new research by Susanne Shultz and her team at Oxford, the ancestors of humans and most other primates likely really did go from solitary to social virtually - and, as it turns out, literally - overnight. Of course, as they themselves point out, "social behaviors do not fossilize", so it isn't necessarily easy to track the deep origins of our own social evolution. They used a mix of modern observations and statistical modelling to give us our best idea yet of where the human capacity for social interaction came from.

Until now, a common explanation for primate social structure is the surrounding environment, in that local food scarcity will force individuals to band together to pool resources and survive. The problem with that is, according to Shultz and her team, is that modern primates live in the exact same social groups regardless of where they live. It's conceivable that ancient primates really did band together because of food scarcity, but if that was the only reason underpinning these complex societies, it seems very strange that primates like baboons and macaques would live together in exactly the same way when they've got plenty of food as those of corresponding populations where there's very little.

Instead, the researchers looked for another mechanism that could cause primates to become more social in as few evolutionary steps as possible. They also built a statistical model to simulate what might happen if the most recent common ancestor of all today's monkeys and apes - a creature that lived roughly 50 million years ago - were to start banding together either in pairs or groups.

To their surprise, the statistical model indicated that it actually made far more sense for this ancient primate to go straight from solitary to loosely affiliated groups of both genders, skipping pairs entirely. That's unlike the evolution of social groups we've seen in other species, but it makes sense when you consider the most likely motivation behind this switch.

Around 52 million years ago, our evolutionary ancestor switched from being a largely nocturnal creature to one that was active during the day. While a creature would want to be on its own at night for maximum sneakiness, a small primate would enjoy safety in numbers during the day as the best way to stay safe from advancing predators. Pairs wouldn't be much use against a large predator, which explains why primates - uniquely, as far as we can tell - went straight to forming large groups.

Of course, not all primates live in such groups - some monkeys, for instance, live in pairs, while gorillas organize themselves in harems with a single adult male and lots of females. The researchers believe both of these innovations came much later, both around 16 million years ago.

The researchers argue that the loose affiliations that sprang up 52 million years ago quickly developed into more stable societies and, in turn, the development of cooperative behaviors. They argue that this evolutionary jump from solitude to society may well have been a key factor in the evolution of many uniquely primate traits, including our own advanced intelligence. So, basically, everything we humans have now might well be because, 52 million years ago, some primates decided they'd had enough of working nights.